1. Field of the Invention
The present invention relates to a power supply system, and more particularly to a power supply system which is capable of supplying DC power by using a battery in the event of a failure of an AC power supply.
2. Description of the Related Art
A power-supplied device which will remarkably benefit from the effects of the present invention is a telephone switchboard, for instance.
In general, the telephone switchboard operates on -48V DC power. This DC current is supplied from a battery storing DC power converted from AC power supplied by an AC commercial power supply.
FIG. 7 shows a conventional power supply system for use with a telephone switchboard. A commercial AC power supply is connected to a rectifier 101. AC power provided by the AC power supply is converted to DC power to charge batteries 102a, 102b. Also connected to the rectifier 101 is a small generator provided for use in the event of a failure of the commercial power supply.
The batteries 102a, 102b have positive electrodes thereof grounded. The batteries 102a, 102b supply DC power at -48V to switchboards 105, 106 via distribution boards 103, 104. The DC power from the distribution board 103 is also supplied to another device 109 which needs the DC power. The distribution board 104 serves exclusively for the switchboards 105, 106 which are provided redundantly. The switchboard 105 is a working switchboard (#0), while the switchboard 106 is a backup one (#1). The switchboards 105, 106 have respective control units (CC) 107, 108 connected thereto.
This type of power supply system has a ground window (GW) 111 formed by a metal piece grounded via a ground line (MGB). A battery ground line (CG) 112, a switchboard ground line (SG) 113, and a cabinet ground line (FG) 114 are connected to this ground window (GW) 111.
Information of a failure of the commercial power supply is sent to an alarm collecting unit 119, which in turn sends the information to the switchboards 105, 106.
The -48V DC power is supplied to still another device 110 via the distribution board 104 and a line 116 connected between the distribution board 104 and the device 110. In the device 110, the DC voltage is converted to another DC voltage by an internal DC--DC converter. The device 110 operates on the voltage obtained by the conversion. A line 117 is a ground line for the new voltage produced in the device 110, while a line 118 is a cabinet ground line.
When this kind of power supply system is used in a developing country, the failure of a commercial power supply frequently occurs, causing wear and degradation of the small generator at a fast rate. Therefore, it is likely that the small generator can no longer operate properly in the event of a failure of the commercial power supply, and the voltage provided by the batteries 102a, 102b is gradually decreased, finally disabling the switchboards 105, 106. In such a case, the switchboards 105, 106 are notified by the alarm collecting unit 119 of occurrence of the failure of the commercial power supply. However, the switchboards 105, 106 are not warned of the danger of being disabled due to a decrease in the voltage provided by the batteries 102a, 102b.
Therefore, hard disk drives provided in the control units 107, 108 can be forced to stop in the middle of writing operation. In such a case, it often happens that a writing arm of the hard disk drive touches a disk surface to blemish the same. Further, there occurs a problem that the working control unit 107 and the backup control unit 108 are forced to terminate operation with non-volatile memories thereof having different data stored therein.
Furthermore, noise generated in the device 109 reaches the ground window (GW) 111 via the battery ground line (CG) 112 and then enters the switchboards 105, 106 via the switchboard line (SG) 113 and the cabinet ground line (FG) 114. Further, noise having a switching frequency is generated in the device 110 during DC--DC conversion and enters the switchboards 105, 106 via the lines 117, 118.
In developing countries, electric works concerning power supplies, which are involved in inside extension of switchboards, takes place frequently. In such electric works, it is likely that grounding work is not properly carried out or proper grounding is not established due to inherent improper connections of reinforcing steels of the building within which the electric works are carried out. Such improper grounding causes mixing of the above-mentioned noise with telephone signals to a significant degree. Further, the frequency of a clock used in the internal circuit of a telephone switchboard is apt to be increased, and the operating voltage in the circuit is apt to be decreased, which leads to a decrease in resistance of the switchboard to entry of the noise.